once, long ago in a basement far, far away, I read that estimated tube life is filament-on hours.

asked an engineer once, being a broadcast brat, and was told for an 8,000 hour tube life estimate, they swapped 'em out at 6,000 hours unless somebody broke the spare set. in the case of image orthicons, they put a tube with a hard burn or at hours into the freezer at home, and retried it a few months later. the cerium oxide target got a little mojo back in the cold and dark.

It is also based on commercial operation of 24/7 where they monitor the number of hours because they don't want to be caught with a failure. So they establish a basis on which to change them from the manufacturer's recommendation. In life critical situations it may be more often than the manufacturer's specs or in a broadcast situation it may be what is specified. In almost all cases when a tube is retired, it works very well in amateur service where the transmit time is 10 mins. on and 20 off.

I think we must also be aware of the idea that those hours are predicted based upon mostly continuous operation as well. (Or at least the practice of leaving filaments ON when transmitter is not on, or only turning the thing fully off when maintenance may be needed...)

I would not expect the intermittent use tube, which would be put through many more heatup and cooldown cycles and the accompanying expansion and contraction such involves, to be able to meet the same spec.

If you 50+ don't worry about tube life unless you are a LID, it/ they will outlast you in phone mode. If your younger then the tube/ tubes will still be fine when you fire up the Boat Anchor to impress your grandchildren.

The RCA data sheet for the 813 (and several other tubes have the same wording) says:

"During standby periods in intermittent operation, it is recommended that the filament voltage be reduced to 80% of normal when the period is less than 15 minutes. For longer periods, the filament should be turned off."

This strikes me as being a little strange, insofar as the the current surge when witching on from cold is not a negligible factor.

In almost all cases when a tube is retired, it works very well in amateur service where the transmit time is 10 mins. on and 20 off.

Off and on service, for the same dissipation, is much more stressful than steady state operation for everything but emission failures. Emission failures, outside tube manufacturing defects, are virtually unheard of in amateur service.

The RCA 813 stuff about switching filament voltage sounds bizarre on the surface. Maybe people were running the filaments too hot, or running the filaments for many hours without transmitting, or they were in a service that vibrated the hot filament. Who knows!

It is interesting when you look at military radios that operated in a vibration prone environment such as the BC610 in the SCR299 (250TH), the BC191 in a B17 (211) and the ART13 in a B29 (813). All filament tubes.....and not switched.

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